(1) Thermal Barrier Coating (TBC)
A high-temperature equipment member, such as an industrial gas turbine blade or a boiler tube, has a coated surface to obtain enhanced heat resistance and corrosion resistance, in many cases. With regard to heat resistance, a ceramics coating, so-called thermal barrier coating (TBC), is typically applied to the equipment member.
In use of ceramics, an alloy layer, so-called undercoat (or bondcoat), is generally inserted between a ceramics layer and a metal substrate to assure adhesion therebetween, because the relatively large difference in thermal expansion coefficient between ceramics and substrate metal is likely to cause peeling of the ceramics layer at the TBC/substrate interface. However, under an ultra-high temperature environment of about 800 to 1200° C., the undercoat will be deteriorated due to reaction with the substrate, or the peeling of the ceramics layer will occur due to an oxide layer thickly grown on the surface of the undercoat. This leads to a serious problem that the equipment member has a short life span of several months.
(2) Al (or Cr, Si) Diffusion Coating Process
With regard to corrosion resistance, the equipment member is typically subjected to an Al (or Cr, Si) diffusion coating process. However, under an ultra-high temperature environment of about 800 to 1200° C., a protective film cannot be stably maintained for a long time-period due to significantly high diffusion rate and high reactivity of a metal element contributing to corrosion resistance. Further, under a severe corrosive environment containing Cl or S, an metal element, such as Cr or Al, constituting the protective film will be quickly consumed even in a temperature range of 500 to 800° C., which leads to difficulties in maintaining the protective film stably for a long time-period, and a serious problem of significantly shortened life span in the equipment member.
(3) Ni—Cr Thermal Spraying
For providing enhanced corrosion resistance, a high Ni-high Cr alloy thermal sprayed coating is applied to the equipment member, in some cases. However, this case involves the same problem as that in the above (2).
(4) TBC System Combined with Re-added Undercoat
Japanese Patent Laid-Open Publication No. 11-61439 discloses a TBC system including an undercoat with Re (Rhenium) added thereto at 12 weight % or more (several % by atomic composition). Japanese Patent Laid-Open Publication No. 2000-511236 titled “Structural Component including Superalloy Substrate and Layer Structure formed thereon, and Production Method thereof” discloses an undercoat for a TBC, containing Re in the range of 35 to 60 weight % (about 15 to 30% by atomic composition). These publications discloses neither the function of Re nor the effect of the Re addition. U.S. Pat. No. 6,299,986 discloses a superalloy article in which a barrier coating containing 4 wt % or less of Re is formed on a Ni-based superalloy substrate containing 5.0 to 7.0 wt % of Re.
(5) Re Coating & Re Alloy Coating
Japanese Patent Laid-Open Publication No. 03-120327 discloses a protective film containing 1 to 20 wt % of Re and 22 to 50 wt % of Cr. Japanese Patent Laid-Open Publication No. 09-143665 discloses a production method of a high-temperature member comprising pure Re, Re—Mo alloy or Re—W alloy. The method disclosed in this publication is intended to produce a Re or Re alloy structural member which is used independently in the form of a thin plate.